In cellular systems for wireless communication, base stations of different kinds are normally used. A typical base station (BS) in a cellular system is often deployed above roof-top level or in a high mast, increasing its coverage area. This gives a high likelihood for line-of-sight between base stations, which can lead to significant BS-to-BS station interference, in particular when a relatively high transmission power is used.
In a Time Division Duplex (TDD) system, uplink data traffic (to a BS) and downlink data traffic (from a BS) use the same frequency carrier, but are separated in time. During a first time period, the downlink data traffic is active, and during a second time period the uplink data traffic is active, the downlink data traffic and uplink data traffic not being active at the same time.
This may cause problem if two cells have different periods where uplink data traffic and downlink data traffic are active, which results in a period of overlap, where uplink data traffic and downlink data traffic are active at the same time, resulting in interference. The interference is also present between mobile stations (MS), such as cell phones, and relay nodes.
In a TDD system, the BS-to-BS and MS-to-MS station interference within the system can be severe since transmission and reception are done on the same frequency. Even if the two cells are located in adjacent carrier frequencies, the interference could be significant.
The interference situation is improved if all base stations synchronize their transmission and reception time windows. Due to propagation delay, guard periods, or silent periods, are needed to allow the transmissions from far away base stations to decay to an acceptable level. Time synchronized interference to neighboring carriers can be suppressed similarly. For the case with two unsynchronized TDD systems transmitting on adjacent frequency bands, the interference in uplink slots from base stations transmitting in downlink on the neighboring carrier can be very strong.
Due to propagation delay over the air, the interference from neighbouring base stations transmitting on the same carrier will be high directly after the switch from DL transmission to UL reception.
In a Frequency Division Duplex (FDD) system, the uplink and downlink activities use different frequencies for their carriers, and thus the BS-BS, and MS-MS, interference is typically highly suppressed by the use of duplex filters and large separation between the uplink and downlink bands.
However, in some cases FDD systems are deployed at frequencies such that the downlink of one FDD system is close to the uplink of the other, e.g. use of both 800 MHz and 900 MHz frequency plans.
Furthermore, in relevant sharing scenarios, FDD and TDD systems may be required to coexist at frequency separations which are much less separated than the typical FDD duplex distance. Additional guard bands and conformance to tight spectrum masks may be required for minimizing interference between the two systems.
BS-to-BS interference can also be reduced by increasing the separation between base stations or by avoiding line-of-sight situations. However, this severely limits the operator choice of deployment alternatives and site locations. Furthermore, in a multi operator scenario, it may not be in one operator's interest to reduce the interference another operator experiences, since they are competitors.
There are a number of drawbacks with the existing solutions to the interference problems discussed:                Guard bands and guard periods represent unused resources and lead to lower spectrum efficiency.        Sharpening the filter requirements only work when the interferer and the victim of the interference are located on different carriers, and even then the filters present physical entities with certain costs and limitations        Increased physical separation is often not a practical solution and severely limits the operators site location choices. If the second system is deployed after the first system, the first systems locations are already in place and choices are limited.        Synchronization in TDD may be difficult or even impossible if there are two different technology TDD systems (e.g WiMax and LTE/TDD) deployed on neighboring frequency bands.        Synchronization between uplink and downlink usage is in general not possible in a sharing situation between a TDD and a FDD system, e.g. WiMax or LTE/TDD on the one hand and LTE/FDD or WCDMA or WCDMA HSPA on the other hand.        
There clearly exists a need to obtain an arrangement that provides reduced interference between transceiver equipment such as mobile station, base stations and relay nodes, which arrangement does not have the drawbacks discussed above.